This disclosure generally relates to semiconductor devices and more particularly to a semiconductor device having a GaN bulk crystal substrate.
GaN is a III-V compound semiconductor material having a large bandgap of blue to ultraviolet wavelength energy. Thus, intensive investigations are being made with regard to development of optical semiconductor devices having a GaN active layer for use particularly in optical information storage devices including a digital video data recorder (DVD). By using such a light emitting semiconductor device producing blue to ultraviolet wavelength optical radiation for the optical source, it is possible to increase the recording density of optical information storage devices.
Conventionally, a laser diode or light-emitting diode having a GaN active layer has been constructed on a sapphire substrate in view of the absence of technology of forming a GaN bulk crystal substrate.
FIG. 1 shows the construction of a conventional GaN laser diode according to Nakamura, S., et al., Jpn. J. Appl. Phys. vol. 36 (1997) pp. L1568-L1571, Part 2, No. 12A, 1 Dec. 1997, constructed on a sapphire substrate 1.
Referring to FIG. 1, the sapphire substrate 1 has a (0001) principal surface covered by a low-temperature GaN buffer layer 2, and includes a GaN buffer layer 3 of n-type grown further on the buffer layer 2. The GaN buffer layer 3 includes a lower layer part 3a and an upper layer part 3b both of n-type, with an intervening SiO2 mask pattern 4 provided such that the SiO2 mask pattern 4 is embedded between the lower layer part 3a and the upper layer part 3b. More specifically, the SiO2 mask pattern 4 is formed on the lower GaN buffer layer part 3a, followed by a patterning process thereof to form an opening 4A in the SiO2 mask pattern 4.
After the formation of the SiO2 mask pattern 4, the upper GaN layer part 3b is formed by an epitaxial lateral overgrowth (ELO) process in which the layer 3b is grown laterally on the SiO2 mask 4. Thereby, desired epitaxy is achieved with regard to the lower GaN layer part 3a at the opening 4A in the SiO2 mask pattern 4. By growing the GaN layer part 3b as such, it is possible to prevent the defects, which are formed in the GaN layer part 3a due to the large lattice misfit between GaN and sapphire, from penetrating into the upper GaN layer part 3b. 
On the upper GaN layer 3b, a strained super-lattice structure 5 having an n-type Al0.14Ga0.86N/GaN modulation doped structure is formed, with an intervening InGaN layer 5A of the n-type having a composition In0.1Ga0.9N interposed between the upper GaN layer part 3b and the strained superlattice structure 5. By providing the strained superlattice structure 5 as such, dislocations that are originated at the surface of the sapphire substrate 1 and extending in the upward direction are intercepted and trapped.
On the strained superlattice structure 5, a lower cladding layer 6 of n-type GaN is formed, and an active layer 7 having an MQW structure of In0.01Ga0.98N/In0.15Ga0.85N is formed on the cladding layer 6. Further, an upper cladding layer 8 of p-type GaN is formed on the active layer 7, with an intervening electron blocking layer 7A of p-type AlGaN having a composition of Al0.2Ga0.8N interposed between the active layer 7 and the upper cladding layer 8.
On the upper cladding layer 8, another strained superlattice structure 9 of a p-type Al0.14Ga0.86N/GaN modulation doped structure is formed such that the superlattice structure 9 is covered by a p-type GaN cap layer 10. Further, a p-type electrode 11 is formed in contact with the cap layer 10 and an n-type electrode 12 is formed in contact with the n-type GaN buffer layer 3b. 
It is reported that the laser diode of FIG. 1 oscillates successfully with a practical lifetime, indicating that the defect density in the active layer 7 is reduced successfully.
On the other hand, the laser diode of FIG. 1 cannot eliminate the defects completely, and there remain substantial defects particularly in correspondence to the part on the SiO2 mask 4 as represented in FIG. 2. See Nakamura S. et al., op cit. It should be noted that such defects formed on the SiO2 mask 4 easily penetrate through the strained superlattice structure 5 and the lower cladding layer 6 and reach the active layer 7.
In view of the foregoing concentration of the defects in the central part of the SiO2 mask pattern 4, the laser diode of FIG. 1 uses the part of the semiconductor epitaxial structure located on the opening 4A of the SiO2 mask 4, by forming a mesa structure MS in correspondence to the opening 4A. However, the defect-free region formed on the opening 4A has a lateral size of only several microns, and thus, it is difficult to construct a high-power laser diode based on the construction of FIG. 1. When the laser diode of FIG. 1 is driven at a high power, the area of optical emission in the active region extends inevitably across the defects, and the laser diode is damaged as a result of optical absorption caused by the defects. Further, the laser diode of FIG. 1 having such a construction has other various drawbacks associated with the defects in the semiconductor epitaxial layers, such as large threshold current, limited lifetime, and the like. Further, the laser diode of FIG. 1 has a drawback, in view of the fact that the sapphire substrate is an insulating substrate, in that it is not possible to provide an electrode on the substrate. As represented in FIG. 1, it is necessary to expose the top surface of the n-type GaN buffer layer 3 by an etching process in order to provide the n-type electrode 12, while such an etching process complicates the fabrication process of the laser diode. In addition, the increased distance between the active layer 7 and the n-type electrode 12 causes the problem of increased resistance of the current path, while such an increased resistance of the current path deteriorates the high-speed response of the laser diode.
Further, the conventional laser diode of FIG. 1 suffers from the problem of poor quality of mirror surfaces defining the optical cavity. Due to the fact that the sapphire single crystal constituting the substrate 1 belongs to hexagonal crystal system, formation of the optical cavity cannot be achieved by a simple cleaving process. It has been therefore necessary to form the mirror surfaces, when fabricating the laser diode of FIG. 1 by conducting a dry etching process, while the mirror surface thus formed by a dry etching process has a poor quality.
Because of the foregoing reasons, as well as because of other various reasons not mentioned here, it is desired to form the substrate of the GaN laser diode by a bulk crystal GaN and form the laser diode directly on the GaN bulk crystal substrate.
With regard to the art of growing a bulk crystal GaN, there is a successful attempt reported by Porowski (Porowski, S., J. Crystal Growth 189/190 (1998) pp. 153-158, in which a GaN bulk crystal is synthesized from a Ga melt under an elevated temperature of 1400-1700° C. and an elevated N2 pressure of 12-20 kbar (1.2-2 GPa). This process, however, can only provide an extremely small crystal in the order of 1 cm in diameter at best. Further the process of Porowski requires a specially built pressure-resistant apparatus and a long time is needed for loading or unloading a source material, or increasing or decreasing the pressure and temperature. Thus, the process of this prior art would not be a realistic solution for mass-production of a GaN bulk crystal substrate. It should be noted that the reaction vessel of Porowski has to withstand the foregoing extremely high pressure, which is rarely encountered in industrial process, under the temperature exceeding 1400° C.
Further, there is a known process of growing a GaN bulk crystal without using an extremely high pressure environment for growing a GaN bulk crystal as reported by Yamane, H., et al., Chem. Mater. 1997, 9, 413-416. More specifically, the process of Yamane et al. successfully avoids the use of the extremely high-pressure used in Porowski, by conducting the growth of the GaN bulk crystal from a Ga melt in the presence of a Na flux.
According to the process of Yamane, a metallic Ga source and a NaN3 (sodium azide) flux are confined in a pressure-resistance reaction vessel of stainless steel together with a N2 atmosphere, and the reaction vessel is heated to a temperature of 600-800° C. and held for a duration of 24-100 hours. As a result of the heating, the pressure inside the reaction vessel is elevated to the order of 100 kg/cm2 (about 10 MPa), which is substantially lower than the pressure used by Porowski. As a result of the reaction, GaN crystals are precipitated from the melt of a Na—Ga system. In view of the relatively low pressure and low temperature needed for the reaction, the process of Yamane et al. is much easier to implement.
On the other hand, the process of Yamane relies upon the initially confined N2 molecules in the atmosphere and the N atoms contained in the NaN3 flux for the source of N. Thus, when the reaction proceeds, the N2 molecules in the atmosphere or the N atoms in the Na—Ga melt are depleted with the precipitation of the GaN crystal, and there appears a limitation in growing a large bulk crystal of GaN. The GaN crystals obtained by the process of Yamane et al. typically have a size of 1 mm or less in diameter. Thus, the process of Yamane et al. op cit., while being successful in forming a GaN bulk crystal at a relatively low pressure and temperature, cannot be used for a mass production of a GaN substrate in the industrial base.